1. Field of the Invention
The present invention relates to a data processing method and apparatus.
2. Description of the Prior Art
Recently, digital display screens such as liquid crystal displays (LCD) have become increasingly popular for viewing broadcast media content such as television programmes and films. Additionally, such displays may be used to view pre-recorded media content stored on a storage medium such as DVD-ROM or Blu-ray® disc. Digital displays typically display images using a progressive scanning method in which scan lines of image pixels are displayed sequentially on the display.
However, broadcast media content is typically broadcast in an interlaced format for display on analogue displays such as CRT (Cathode Ray Tube) displays. Analogue displays typically use an interlaced scanning method in which odd and even lines of image pixels are displayed consecutively.
In order to display the media content on a digital display, such as an LCD, the interlaced media content may need to be interpolated so as to generate interpolated pixels for display between the odd and even lines respectively. Additionally, where the media content has a different image resolution from that of the display on which the content is to be displayed, image interpolation may need to be carried out so as to generate a scaled image.
For example, where a television programme is broadcast in standard definition (SD) and is to be displayed on a high definition (HD) display, image interpolation can be carried out between input pixels of the standard definition television programme so as to generate output pixels for display in the high definition format. Alternatively, where content to be displayed has a higher resolution than the display on which the content is to be displayed, decimation of pixels may be carried out so as to shrink the image for display at the lower resolution.
There are several known methods for carrying out image interpolation. However, some types of image may cause image artefacts to be produced when using known interpolation methods. One such type of image is one comprising a discontinuity in pixel value. For example, a discontinuity may be where there is a sudden transition from a black pixel to a white pixel.
In a known method of image interpolation, a bi-cubic filter is applied to input pixels so as to generate the interpolated pixels. However, bi-cubic filters are unsuitable for performing image scaling operations, especially shrinking operations, as bi-cubic filters perform a decimation operation rather than a filtering operation. This may be addressed by using a sinc filter to generate the interpolated pixels so that both image shrinking and expanding operations can be performed.
However, in some cases, the use of a sinc filter may cause an image artefact termed ringing to become apparent in the output image. Ringing typically occurs where there is a discontinuity in the image such as that corresponding to the edge of an object. A sinc filter is a slowly decaying periodic filter and, consequently, when the filter is applied to source pixels, the filter characteristics may become visible in the output image. Accordingly, ringing may manifest itself in the output image as a series of periodic image features near the discontinuity. A detailed example of ringing will be described in more detail later. Although this problem may be reduced by using different length sinc filters or windowed filters, ringing may still be apparent in the output image.
US-A-2005/0286804 discloses a method of image interpolation for reducing ringing when using a sinc filter to generate interpolated source pixels by detecting frequencies in the source pixels and carrying out a weighted clip of the filter output if a high frequency is detected in the source pixels. However, this system may not work well when shrinking an image, and may only reduce ringing in some images.
Although a sinc filter can be used to perform image scaling, the use of a sinc filter to generate interpolated pixels may lead to the appearance of image artefacts in the form of ringing in the output image.